I am confused about the conceptual relationship/differences and usage scenarios between tox and Travis-CI.
I saw Python projects are testing for different Python environments by using tox OR Travis-CI respectively. So it seems that these two CI services can be the alternative to each other.
But I also read that we can run tox with Travis-CI. And this seems that these two CI services are the complementary to each other.
So I am wondering why do we still need Travis-CI if we already have tox (Or the other way around)? Hope someone can make clarification or point some resources about the conceptual relationship and/or differences and usage scenarios between these two? Thanks in advance!